The present invention broadly relates to treatment of subterranean formations, especially those bearing hydrocarbons. More particularly, the present invention relates to a method of treating such formations with a particulate material coated with a phenolic resin.
Subterranean formations surrounding oil wells, gas wells, water wells, and other similar bore holes, are frequently treated with particulate materials in the form of sand or glass beads to alter the producing characteristics of the particular well. The type of treatments vary but can include hydraulic fracturing, sand consolidation, or gravel pack completion. All of these techniques, however, utilize particulate material that is injected into the well and into the geological formation surrounding and adjacent the bore hole. Commonly, the particulate material is coated with a thermosetting synthetic resin which is cured in situ in the formation to bond the particulate matter together once it is injected into the formation.
One thermosetting resin utilized in the past is a one-step, or resol, phenolic resin. Generally, the resol resin is coated or mulled onto particulated material such as well sand, either with or without the presence of a solvent. The coated sand is then injected into the well and thence into the geological formation surrounding the well. The resol resins are designed so that the heat naturally present in the geological formation is sufficient to cure the resin in situ in a period of from a few days up to 60 days or more. The resulting resin-bonded subterranean structure exhibits relatively high compressive strength and permeability, two characteristics that are required for effective treatment of subterranean formations.
Although well sand can be mulled with a resol resin when hot and in the absence of a solvent, the resulting coated resin becomes partially cured because of the required elevated mulling temperature and thus, when injected into a well, exhibits lower compressive strength and generally lesser permeability. It is therefore most desirable with resol resins to first dissolve them in a resin solvent and then coat the sand with the solvent-borne resin or to incorporate a solvent during the coating procedure. Mulling in the presence of a solvent, however, creates much longer mixing times than when mulling in the absence of a solvent.
Once sand is coated with a resol resin, it has a limited storage life. The resol resin tends to partially cure on the surface of the coated particles. Thus, when the resin is cured, in situ, the bond formed between particles is lower in strength. Furthermore, sands coated with solvent-borne resin tend to retain a small amount of solvent in the final product. The retained solvent gives the coated particles a sticky surface causing them to adhere to each other, thus preventing free flow of the particles when transferring them from one location to another and when injecting them into a bore hole. Lastly, solid resol resins require special manufacturing techniques, are harder to handle because they are inherently thermally unstable, and because the solid resols are constantly curing, the quality of the final cured resin is always unpredictable.
It has been suggested that two-step novolac-type phenolic resins be substituted for the one-step resol phenolic resins to cure some of these problems. Attempts to make this substitution, however, have always resulted in a coated sand having a lower compressive strength than that achievable with resol resins. Heretofore, it has been thought that the reduced strength of the conventional novolac resin-bonded sand was caused by the loss of polymerization catalyst hexamethylenetetramine (hexa) into the formation before the resin could cure. Increasing the amount of hexa in the uncured novolac has not, however, led to significant increases in compressive strength.
It is accordingly a broad object of the present invention to provide an adhesive for bonding particulate matter together in situ in subterranean formations, which adhesive has the favorable characteristics of one-step resol-type phenolic resins, but which does not exhibit the handling, short life, stability and mixing problems associated with resol-type resins. It is a further object of the present invention to define and identify a two-step novolac-type resin that has a compressive strength equal to that of the one-step resol-tyope resins when utilized in a subterranean formation to bond particulate matter together.